The invention pertains to a current collector made from ferritic iron alloy for electrically connecting and mechanically supporting a set of individual, planar SOFC high-temperature fuel cells (solid oxide fuel cells). The fuel cells comprise an anode, an electrolyte, and a cathode, operate at temperatures of between 700xc2x0 C. and 900xc2x0 C., and are equipped with a solid electrolyte.
In recent years, SOFC high-temperature fuel cells have experienced considerable progress in development and are beginning to become economically viable. The SOFC-type fuel cell is wherein by a plate-like structure and a solid oxide ceramic electrolyte. Different oxide ceramic electrolytes, for example doped zirconium oxide (zirconia) or cerium oxide (ceria), are used depending on the working temperature selected for the cell in the range between 500xc2x0 and 1000xc2x0 C. The cell voltage of an individual fuel cell is approximately 1 volt, and therefore it is always necessary for a multiplicity of individual cells with surface dimensions which are as large as possible to be stacked and electrically connected in series in order to achieve electrical voltages and power outputs that are technically useful.
In actual fact, nowadays plate-like fuel cell arrangements with a surface area of up to 1000 cm2, wherein the thickness of the electrodes and of the solid electrolyte is regularly much less than 100 xcexcm, are used. The lowest possible electrolyte thickness, which is important for the efficiency of the cell, is between 5 and 30 xcexcm. In this context, a distinction is drawn between unsupported and supported electrolytes, e.g. of the ASE (anode supported electrolyte) type. Plate-like individual cells of this type stacked on top of one another are separated from one another by so-called current collectors, also known as connecting elements, interconnectors, or bipolar plates. The cells are mainly supplied with the required fuels and the reactive media are removed, and the cells are at the same time also mechanically stabilized, by means of open distribution passages in the current collectors.
It is therefore quite understandable that the development of suitable current collectors has in recent years been the subject of considerable attention, both with regard to the selection of material and with regard to economic fabrication thereof to form complex components. The complexity of the components is primarily determined by the generally filigree, open passage and line systems used for the gaseous media.
To be satisfactorily useable over the entire fuel cell service life, which has to be sufficiently long from an economic viewpoint, the current collectors have to meet high demands imposed on a wide range of mechanical, physical and chemical material properties and at the same time it must be possible to manufacture the current collectors at relatively low cost. The material costs alone must not make the overall fuel cells system commercially unattractive.
The indispensable high material quality demands relate to:
high mechanical strength, in particular high rigidity of even thin current collector plates over the wide temperature range between room temperature and approx. 1000xc2x0 C.
optimum matching of the coefficient of thermal expansion to that of the solid electrolyte film: this match must be equally present at any temperature in the entire range between room temperature and working temperature.
high thermal and electrical conductivity, low electrical surface contact resistance, including maintaining these values throughout the entire service life of a fuel cell.
high corrosion resistance of the material with respect to the fuel gas and exhaust gas atmospheres in the cell, which on the anode side are substantially hydrogen and H2O vapor, CO and CO2, and on the cathode side are substantially oxygen and air.
The development of suitable materials for current collectors was initially concentrated on chromium alloys. In recent years, the development concentration has shifted to ferritic iron alloys with significant levels of chromium.
During the efforts to further refine the proposed ferritic alloys for current collectors in SOFC-type fuel cell units, it has been important to suppress the formation of volatile chromium compounds and the vaporization of these compounds from the current collector surface as far as possible. By way of example, one countermeasure proposed has been the addition of suitable quantities of titanium and manganese.
Even with the ferritic materials, which are known to be resistant to corrosion, it has been impossible to completely avoid superficial growth of oxide. To reduce the oxide growth rate, but at the same time also to increase the mechanical strength, it has been proposed to add small quantities of the elements yttrium, cerium, lanthanum, zirconium and/or hafnium.
With materials developments of this type, the person skilled in the art has been relying on the theoretical and empirical knowledge of the action of individual metallic and nonmetallic components. Known ferritic iron-based materials with a multiplicity of additions which have by now been described, in view of the state which has been reached in the demands for matching a wide range of extremely divergent materials properties, make a prediction about measures aimed at further matching of properties impossible or at least rather dubious.
The validated prior art forms an important platform but not a reliable indicator toward materials developments of this nature.
For example U.S. Pat. No. 6,156,448 (European patent EP 0 880 802 B1) describes a high-temperature fuel cell with stabilized zirconia as solid electrolyte, wherein the current collectors consist of an iron-based alloy comprising 17 to 30% by weight of chromium, such that this material has a coefficient of thermal expansion of between 13 and 14xc3x9710xe2x88x926Kxe2x88x921.
A material that is characterized in this way for current collectors has no guiding significance in the context of this description with regard to matching of properties. Even with regard to the coefficients of thermal expansion, nowadays more refined criteria apply, for example in connection with the design and material of the solid electrolyte used in each case.
U.S. Pat. No. 5,800,152 (European published patent application EP 0 767 248 A1) describes an oxidation-resistant, metallic material, in particular also for use in current collectors for high-temperature fuel cells, of the following composition: 15 to 40% by weight of chromium, 5 to 15% by weight of tungsten, 0.01% to 1% by weight of one or more elements selected from the group consisting of Y, Hf, Ce, La, Nd and Dy, remainder iron, which material has a coefficient of thermal expansion of more than 12xc3x9710xe2x88x926 and less than 13xc3x9710xe2x88x926Kxe2x88x921 in the temperature range between room temperature and 1000xc2x0 C.
As an alternative, this material must additionally contain 0.001 to 0.01% by weight of boron.
The document states that this material is specifically designed for use in combination with zirconium oxide as solid electrolyte at working temperatures of between 900xc2x0 C. and 1000xc2x0 C.
An article by the two inventors of the noted patent which was published after the priority date of this description (M. Ueda, H. Taimatsu, Thermal Expansivity and High-Temperature Oxidation Resistance of Fexe2x80x94Crxe2x80x94W Alloys Developed for a Metallic Separator of SOFC, 4th European SOFC Forum Lucerne, Jul. 10-14, 2000) provides a very critical report on difficulties and drawbacks of the said material as a current collector. Alloys containing more than 18% by weight of chromium are considered to be difficult to process. The report refers to layers which are formed on the material as a result of corrosion and which flake off.
Despite tests using the Cr and W contents over the entire range covered by the scope of protection of the alloy, it was impossible for the coefficient of thermal expansion of the alloy to be satisfactorily matched to the coefficient for yttrium-stabilized ZrO2 solid electrolytes. According to new measurements, in the temperature range between 20xc2x0 and 1000xc2x0 C. this material constant varies continuously between 11.7, 10.8 and back to 11.7xc3x9710xe2x88x926Kxe2x88x921. The resistance to oxidiation, in particular under the hot H2/H2O vapor atmosphere which is present on the anode side when the cell is operating was recorded to be unsatisfactory.
It is accordingly an object of the invention to provide a ferritic material for current collectors in high-temperature fuel cells with a solid electrolyte, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which better matches the high and wide-ranging demands on properties referred to in the introduction than prior art materials. The material is in particular to have a better resistance to corrosion under the fuel gas and exhaust gas atmospheres. At the same time, there is to be an electrical contact resistance which is as constant and low as possible between current collectors and cell electrodes over long periods, when these components are in surface-to-surface contact with one another in the cell arrangement. Furthermore, the material is to have a coefficient of thermal expansion that is well matched to that of known SOFC solid electrolytes and electrode materials for medium-temperature fuel cells in the working range between 700xc2x0 C. and 900xc2x0 C.
With the foregoing and other objects in view there is provided, in accordance with the invention, a current collector made from ferritic iron alloy for electrically connecting and mechanically supporting a stack of individual, planar SOFC high-temperature fuel cells having an anode, an electrolyte, and a cathode, and operating at temperatures of between 700xc2x0 C. and 900xc2x0 C., and being equipped with a solid electrolyte. According to the improvement, the ferritic material comprises:
more than 68% by weight of Fe and standard impurities;
22 to 32% by weight of Cr;
1 to 10% by weight of Mo; and
0.01 to 1.5% by weight of at least one material selected from the group consisting of yttrium, rare earth metals, and oxides thereof.
In accordance with an added feature of the invention, the solid electrolyte consists of cerium oxide doped with Gd, Ca, Sm, and/or Y.
In accordance with an additional feature of the invention, the solid electrolyte consists of zirconium oxide doped with Y, Ca, Sc, and/or Yb.
In accordance with another feature of the invention, the current collector is formed in a powder metallurgy process in a shape near net shape, i.e., the green body is produced to near final shape.
In accordance with a further feature of the invention, the ferritic material additionally includes 0.1 to 3% by weight of Nb, Ti, Ni, and/or Mn.
In accordance with a specific embodiment of the invention, the ferritic material consists of 22% by weight of Cr, 2% by weight of Mo, 0.3% by weight of Ti, 0.5% by weight of Y2O3, remainder iron.
In accordance with another specific embodiment, the ferritic material consists of 26% by weight of Cr, 2% by weight of Mo, 0.3% by weight of Ti, 0.5% by weight of Y2O3, remainder iron.
In accordance with a concomitant feature of the invention, the ferritic material consists of 26% by weight of Cr, 2% by weight of Mo, 0.3% by weight of Ti, 0.4% by weight of Nb, 0.5% by weight of Y2O3, remainder iron.
In further summary, the objects of the invention are achieved by a current collector which consists of a ferritic material that, in addition to more than 68% by weight of Fe and standard impurities, also includes 22 to 32% by weight of Cr, 1 to 10% by weight of Mo and 0.01 to 1.5% by weight of yttrium and/or rare earths and/or oxides thereof.
The ferritic iron material according to the invention easily satisfies all the property requirements which have been mentioned above for SOFC high-temperature fuel cells.
The coefficient of thermal expansion of the material according to the invention is well matched to that of oxidic solid electrolyte materials which are currently standard for high-temperature fuel cellsxe2x80x94in particular to gadolinium-stabilized cerium oxide, which is used as electrolyte in the working range from 700xc2x0 C. to 900xc2x0 C., with a coefficient of thermal expansion of 12.5xc3x9710xe2x88x926Kxe2x88x921 at 800xc2x0 C., 12.7xc3x9710xe2x88x926Kxe2x88x921 at 900xc2x0 C. The coefficient of thermal expansion, which varies as a function of temperature, matches that of oxidic solid electrolytes which are customarily used with a rating of very good to satisfactory at any temperature between room temperature and 900xc2x0 C.
The rating xe2x80x9cvery good to satisfactoryxe2x80x9d takes account of the compromise which may have to be reached with regard to optimization of different materials properties, such as coefficient of thermal expansion and electrical contact resistance.
As described below, the alloy according to the invention is predominantly aimed at achieving a minimum level of compromise with regard to the corrosion characteristics and the associated electrical contact resistance at the surface of the material.
Moderate compromises with regard to the coefficient of thermal expansion of the current collector nevertheless lead to unrestricted functioning of the cells if the ASE technique, which is nowadays customary, is used, in the following way.
The solid electrolyte film with a low thickness is no longer unsupported, but rather the solid electrolyte is applied as a supported film directly to an electrode surface as support material, for example as ASE (anode supported electrolyte) composite component. The thinner a film, the more elastic it becomes. In this way, different coefficients of thermal expansion between the cell components which rest against one another with surface-to-surface contact are compensated forxe2x80x94to a limited extentxe2x80x94without there being a risk of the film tearing.
In addition to the good matching of the coefficients of thermal expansion, which is indispensable for current collector materials, the most significant advantage of the ferritic iron material according to the invention compared to the prior art is a surprising and unforeseeable high resistance to corrosion with respect to fuel and exhaust gas atmospheres in the temperature range from 700xc2x0 C. to 900xc2x0 C., with the simultaneous formation of advantageous oxidation products in the surface region of the ferritic material.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a current collector and a ferritc material for a current collector in a high-temperature fuel cell, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The invention will be better understood with reference to the following figures that show the oxidation behavior of configurations of the ferritic iron-based alloy according to the invention compared to ferritic steels which have previously been described for current collectors but also compared to a chromium-base alloy (abbreviation: CRF) which has been in widespread use for current collectors.